Pumps & Systems, March 2013
A few years ago, I took a class to learn to be a horologist—a person who repairs antique mechanical clocks. My instructor said that some of the students may learn to be good clock repair people, but time will tell. That expression started with clock repair.
If a horologist does a quality job repairing a wall clock, it can be wound and will run for eight days before a rewind is needed. If the horologist repaired the clock poorly, the clock might quit after only four days and keep bad time. The device can be rebuilt, but the quality of the time it keeps reflects on the horologist. Do a bad job, and time tells on you.
In a tighter economy, plants and facilities rebuild their own pumps to cut costs. Some plant managers believe that using plant personnel instead sending it out for repair is less expensive. Time will tell with pumps rebuilds, too.
I have participated in the supervision of many electrical and mechanical repairs. If an electrical mistake is made, it is known quickly. The lights go out; sparks fly; or even worse, something melts.
However, mechanical repair mistakes are less obvious. Miss a critical step in alignment, ignore the bearing fit or allow the shaft to wobble, and the pump continues to run. It pumps, but time will tell how long it will last. The unit might only last two years when it should have lasted 10 years. After two years, supervisors may forget who previously serviced the pump, and two years may become the normal expectation. If it stopped working after a week, management may pay more attention. Rebuilding a pump to last requires skill and a focus on detail. This article provides steps to follow when rebuilding a pump to lengthen the time interval between repairs.
Check for Piping Strain
A simple check can be made to avoid piping misalignment and the strain that it puts on critical pump components, such as bearings and mechanical seals. When the pump has been properly shut down with safety locks in place, separate the coupling between the pump and the driver.
Place two dial indicators on the pump coupling half or the pump shaft if it is accessible. A good way to hold them in place is with a magnet base. One indicator is placed on the side of the coupling half to detect horizontal movement. The other is placed at the top to detect vertical movement.
Depress the indicators and set them at zero. Then release the bolts on the suction and discharge flanges. They do not have to be removed, just backed off to be finger tight. If either indicator moves 0.001 inch or more, piping strain exists and must be corrected before re-installing the pump after the rebuild.
This step is often skipped. Chances are the new or rebuilt pump put in the same location with the same piping will have a short life because of the twist and stress induced when the piping is secured.
Inspect Bearing Fits
Installation of anti-friction bearings involves some measurement steps. The bearing is round when it is removed from the box. It has certain prescribed internal clearances that allow for smooth movement. If the shaft is oversized or slightly tapered in the bearing seat or if the housing bore is “belled out,” the bearing will not remain round during operation. Roundness in a bearing means long life.
Check the housing bores and shaft seats with a micrometer capable of reading to ten-thousandth of an inch (0.0001 inch) to ensure proper fit. This step must be completed. If an end user does not know the proper dimensions, they should ask the pump vendor for a “critical dimension checking print,” or look up the proper fit using Machinery Handbook or a similar industrial reference.
Unfortunately, if a machinist fails to make a shaft correctly, he/she will often leave it slightly oversize and the housing bore a bit undersize. Metal is easier to remove than to add back, so they shoot high on the shaft and low on the bore (always leaving metal that can be removed). They sometimes leave more than desired. The pump bearing will install, but it will be pinched on the outside diameter (OD) or expanded too much on the inside diameter (ID) and will fail quickly. Remember, the pump bearing might last a year in this condition, but it should have lasted 10 or more if the dimensions were correct.
Trimming the impeller diameter to give a better performance, even if completed on a lathe, removes unequal amounts of metal from the cast surfaces resulting in dynamic unbalance.
Ensure Pump Shaft Straightness
Pump shafts are subjected to unbalanced impellers, worn bearing fits, impeller rubs and other mechanical strains that can cause them to bow. With the pump shaft removed from the pump during the rebuild and all other components removed, end users should take the opportunity to check the shaft for straightness.
A pool player takes the cue, lays it on the pool table and rolls it back and forth. If it bumps as it rolls, it is not straight. A similar check on a shaft can be completed on a shop bench with a dial indicator and two V-blocks.